1. Field of the Invention
The present invention relates to an image-forming apparatus which forms an image by irradiation of an electron beam onto an image-forming member from an electron-emitting device. The present invention also relates to a method for setting (or designing) preliminarily the electron beam diameter on the image-forming member in production of the image forming apparatus.
2. Related Background Art
Flat panel display apparatus practically used includes liquid crystal display apparatus, EL display apparatus, and plasma display panels. These are not satisfactory for image displaying in view of the visual field angle, displayed colors, luminance, and so forth. In particular, the flat panel display apparatus are inferior to cathode ray tubes (CRT) in the displaying characteristics, and cannot be used as a substitute for the CRT at present.
However, with the progress of information processing by computers, and with the improvement in image quality in TV broadcasting, demands are increasing for the flat panel display apparatus of high definition and large display size.
To meet the demands, Japanese Patent Appln. Laid-Open Nos. 58-1956 and 60-225342 disclose flat panel image forming device which comprises a plurality of electron sources arranged in one plane and fluorescent targets counterposed thereto for receiving an electron beam respectively from the electron sources.
These electron beam display apparatuses have a structure shown below. FIG. 11 illustrates schematically an apparatus constituting a conventional display apparatus. The apparatus comprises a glass substrate 71, supports 72, electron-emitting regions 73, wiring electrodes 74, electron passage holes 14, modulation electrodes 15, a glass plate 5, a transparent electrode 6, and an image-forming member 7. The image-forming member is made of a material which emits light, changes its color, becomes electrically charged, or is denatured on collision of electrons, e.g., a fluorescent material, a resist material, etc. The glass plate 5, the transparent electrode 6 and the image-forming member 7 constitute a face plate 8. The numeral 9 denotes luminous spots of the fluorescent member. The electron-emitting region 73 is formed by a thin film technique and has a hollow structure without contacting the glass plate 71. The wiring electrode may be made of the same material as the electron-emitting region or a different material therefrom, and has generally a high melting point and a low electric resistance. The support 72 may be made of an insulating material or of an electroconductive material.
In such an electron beam display apparatus, a voltage is applied to the wiring electrodes to emit electrons from the electron-emitting regions 73, the electrons are derived by applying a voltage to the modulation electrodes 15 which conduct modulation in accordance with information signals, and the derived electrons are accelerated to collide against the fluorescent member 9. The wiring electrodes and the modulation electrodes are arranged in an X-Y matrix to display an image on the image forming member 7.
The aforementioned electron beam displaying apparatus, which uses a thermoelectron source, has disadvantages of (1) high power consumption, (2) difficulty in display of a large quantity of images because of low modulation speed, and (3) difficulty in display of large area because of variation among the devices.
An image-forming apparatus having arrangement of surface conduction electron-emitting devices in place of the thermoelectron source is expected to offset the above disadvantages.
The surface conduction electron-emitting device emits electrons with a simple structure, and is exemplified by a cold cathode device disclosed by M. I. Elinson, et al. (Radio Eng. Electron Phys. Vol. 10, pp. 1290-1296 (1965)). This device utilizes the phenomenon that electrons are emitted from a thin film of small area formed on a substrate on application of electric current in a direction parallel to the film face.
The surface conduction electron-emitting device, in addition to the above-mentioned one disclosed by Elinson et al. employing SnO.sub.2 (Sn) thin film, includes the one employing an Au thin film (G. Dittmer: "Thin Solid Films", Vol. 9, p. 317 (1972)), the one employing an ITO thin film (M. Hartwell, and C. G. Fonstad: "IEEE Trans. ED Conf.", p. 519 (1975)), the one employing a carbon thin film (H. Araki et al.: "Sinkuu (Vacuum)", Vol. 26, No. 1, p. 22 (1983)), and so forth.
These surface conduction electron-emitting devices have advantages of (1) high electron emission efficiency, (2) simple structure and ease of production, (3) possibility of arrangement of a large number of devices on one substrate, (4) high response speed, and so forth, and are promising in many application fields.
FIG. 12 illustrates construction of an image forming device employing such a surface conduction electron-emitting device in an use for image forming apparatus. The device comprises an insulating substrate 1, device electrodes 2, 3, and electron-emitting regions 4.
In this image-forming apparatus employing the surface conduction electron-emitting devices also, an image is formed by application of a voltage through device wiring electrodes 81 between the device electrodes 2, 3 to emit electrons and by control of the intensity of the electron beam projected to a fluorescent member 7 by applying a voltage to modulation electrodes 15 corresponding to information signals.
As well known, when a planar target is placed in opposition to a thermoelectron source and electrons are accelerated by application of a positive voltage to the target, the electron beam collides against the target in a form corresponding nearly to the shape of the electron source. Accordingly, in an image-forming apparatus employing thermoelectron sources as shown in FIG. 11, the shape of the electron beam spot formed on the image-forming member can readily be controlled by suitably designing the shape of the electron sources. However, the image-forming apparatus employing thermoelectron sources has disadvantages mentioned above and cannot meet satisfactorily the demand for high picture qualities and a large picture size.
On the other hand, the surface conduction electron-emitting device which has the aforementioned advantages is expected to enable the construction of image-forming apparatus which satisfies the above demands. In the surface conduction electron-emitting device, a voltage is applied to the electrodes connected to a thin film in the direction parallel to the substrate surface to flow an electric current in a direction parallel to the thin film formed on the substrate, whereby electrons are emitted. The emitted 10 electrons are affected by the electric field generated by the applied voltage. Thereby the electrons are deflected toward the higher potential electrode, or the trajectory of electrons is distorted before the electrons reach the face of the image-forming member. Therefore, the shape and the size of the electron beam spot on the image-forming member cannot readily be predicted. It is extremely difficult to decide the application voltage (V.sub.f) to the electron-emitting device, the electron beam acceleration voltage (V.sub.a) applied to the image-forming member, the distance (d) between the substrate and the image-forming member, and so forth.
Since the electron beam is subjected to the aforementioned deflecting action during projection onto the image-forming member, the shape of the electron beam spot on the image-forming member will be deformed or distorted, so that a spot in an axial symmetry, like a circle, cannot readily be obtained.